The answer to 'replacement not heating' is that there are several common causes for a replacement HVAC system not heating properly, including issues with the thermostat, furnace, or ductwork. Symptoms may include the system not turning on, blowing cold air, or not reaching the desired temperature.
Some DIY troubleshooting steps you can try include checking the thermostat settings, replacing the air filter, and inspecting the furnace for any visible issues. However, if the problem persists or you're unsure of the cause, it's best to call in a professional HVAC technician to diagnose and repair the issue properly.